core: Update COW preallocated images to COW sparse

Update the volume type of COW images that were imported from export domain
to a block storage domain from preallocated to sparse since COW preallocated
is not not a legal image configuration

Change-Id: Ia50e78021369b6143c028af08742521bbdeb07fe
Bug-Url: https://bugzilla.redhat.com/1132084
Signed-off-by: Tal Nisan <tnisan@redhat.com>